The present invention relates to the protection of conductive cables and more particularly to the protection of cables carrying both signal traffic and AC power.
Coaxial cable has been installed extensively by cable television (CATV) operating companies to bring multi-channel wideband television services to homes. These services have provided almost exclusively downstream signal transmission to the customer with little or no upstream or interactive communication from the customer to the head end. With deregulation of the cable television and telephone industries, opportunities exist to provide two way data and telephone services over the cable television coaxial cable network.
To provide new services over the coaxial cable distribution network, power for operating the circuits at the customer end is required. Power could be derived from the electric power system, but this has the disadvantage of connection to mains power and the need for a backup battery in case of power failure. It is therefore preferable to power the home equipment independently from the drop cable to the house.
As the majority of homes in suburban areas have a coaxial service cable already provided it is economically preferable to use the existing drop to also provide the home circuit powering. It has been determined that the power supplied over the coaxial cable should be in the at voltage levels of 90 V AC or more to ensure adequate powering over a typical CATV distribution system. A this voltage level, electrical codes require that a buried service entrance cable be buried at a depth of 18 inches or more to limit exposure to hazardous voltages. A significant portion of the buried CATV coaxial cable drop cables are buried at depths less than the minimum requirement.
A ground fault protection device is required to ensure safe operation without having to replace the existing drop cable. This poses a unique problem as the outer conductor of the coaxial cable is normally grounded and there is no simple means to distinguish between a power load from the center conductor to ground and an unintended fault to ground along the drop cable.
One solution to this problem is disclosed in the applicant""s prior U.S. Pat. No. 5,793,590, the disclosure of which is incorporated herein by reference. That patent discloses a coaxial drop cable safety system in which tap end and premise end units are connected to the drop cable. The tap end unit applies a DC voltage to the center conductor of the drop cable. A monitoring circuit in the tap end unit monitors the center conductor DC voltage to a fault from the center conductor to ground along the drop cable. The tap unit will immediately remove the powering voltage In the event of either an open or faulted condition on the center conductor along the entire length of the drop cable.
In the prior art system, each of the tap end and premise end units includes a DC blocking capacitor in the AC conducting path. These must be of sufficiently large value that the AC impedance is low. This will keep the AC voltage drop across the capacitors small so as not to reduce significantly the supplied AC powering voltage at the premise end.
With this system, in order to detect a resistive fault to ground, the voltage to ground must fall to a determined minimum trip level. The DC blocking capacitors must discharge through a fault resistor before the trip level can be reached and the disconnect activated. The larger the value of the capacitors, the longer it will take to discharge them and drop the DC voltage to the trip level.
This may be a problem with an application requiring the powering of a Multiple Distribution Unit where several telephones and other powered devices are connected to a single drop. In this case, the DC blocking capacitor must be of a larger value than is required for a single distribution unit. The requisite large capacitors must discharge through the fault resistance. For fault resistances in the order of 20 to 30 thousand ohms, it can take one half second or more to discharge the capacitors to the trip level for the monitoring circuit.
The present invention addresses the question of providing a shorter disconnect time.
According to the present invention there is provided a cable protection system for a cable having first and second conductors for carrying an electric signal and AC power from a source to a destination, the system comprising:
a first circuit component for passing the signal to the first conductor;
a second circuit component with an active state for passing the AC power to the first conductor and a blocking state for selectively blocking the passage of the AC power to the first conductor;
monitoring means including:
means for applying a DC reference voltage to the first conductor; and
means for monitoring DC current in the first conductor; and
actuation means for actuating the second circuit component to the blocking state in response to detection by the monitoring means of a value of said DC current representing a faulted condition of the first conductor.
The system thus removes the powering voltage In the event of a faulted condition on the first conductor, which with a coaxial drop cable will be the center conductor. The actuation to a blocked state is based on a rapid current change rather than a change of voltage that will be delayed by the discharge of large blocking capacitors. The preferred system has a range or xe2x80x9cwindowxe2x80x9d of acceptable DC current levels. Currents outside the window, either above or below the acceptable range, trigger the blocking state.
The monitoring means may include means for applying a DC voltage to a resistive circuit including resistances at opposite ends of the center conductor. This establishes the reference DC current in the center conductor. Variations from this reference value indicate a faulted or open circuit condition of the conductor. The DC circuit is limited to the drop cable by the blocking capacitors so that the system can distinguish between a normal end located load and a fault from the center conductor to ground along the drop cable.